1. Field of This Invention
This invention relates to a storage element for a digital permanent storage (memory) system, which has as an essential component a field effect transistor the semiconductory layer of which is changeable in its electrical conductivity, depending on the state of the storage of a storage medium.
2. Prior Art
A fairly large number of physical phenomena have been known upon the basis of which one could build systems for the purpose of digital storage of information. Some such systems have already been realized. All systems for erasable mass storage systems used in practice hitherto are based however on the principle of magnetic remanence. Within the last few years, a series of new storage methods have been described which have been somewhat intensively developed. Every one of these technological systems has its special advantages and attractive features, which predestines it for special areas of application. However, these advantages are countered by disadvantages, such as, special working conditions (e.g., low temperatures), special methods of operation (e.g., shift registers) or the necessity of extensive external apparatus for control (e.g., optical storage systems).
The possibilities resulting from physics and technology of a part of these storage methods suggest a subdivision into mass storage systems and rapid storage systems is regard to their areas of application. The former generally speaking are holographic systems which have the goal of random access to quantities of date of more than 10.sup.10 bits without intermediate mechanical movement. Rapid storages on the other hand in most cases are integrated semiconductor systems, the access and transfer times of which lie within the nanoseconds range. Generally speaking however, these are not permanent storage systems so that their area of application lies more in the area of the short time storage of relatively small quantities of data.
Between these two extremes, there are a number of processes in development which are projected to have the storage capacities of about 10.sup.8 bits and which have operating times of the order of magnitude of 1 .mu.s[R.E. Matick, Proc. IEEE 60, 3, p. 226 (1972)]. Development of the so-called "magnetic bubble" storage system is quite advanced, which in the case of use of permanent magnets is a genuine permanent storage. One disadvantage of such storage system is the shift register operation. In the case of construction of fairly large storage systems, the length of the register must be relatively large for no other reason than because of the cost. In the case of shift frequencies of from 0.1 to 4 .times. 10.sup.6 bits, average access times of 0.1 to 1 ms result ]F. Perzefall, Elektronic 2, p. 39 (1974)]. Furthermore, to mention a few, the semipermanent MNOS-system [K. E. Lundstrom, and C. M. Svensson, IEEE Trans. of Electronic Devices 19, 6, p. 826 (1972)] and storage systems based on the principle of electronic delay lines [R. M. White, Proc. IEE 58, p. 1238 (1970)] are also under development.
In the case of optically addressed storages systems which utilize, for example the Faraday or Kerr effect on ferromagnetic or ferroelectrical material, there is a lack of suitable light deflectors which cannot approach a sufficiently large number of discrete positions without mechanical intermediate movement.
Several years ago, storage elements were described which were founded on the basis of ferroelectricity. [J. L. Moll and Z. Tarni, IEEE Trans. Electr. Devices vol. ED-14, No. 10, p. 338 (1963); S. S. Perlman and K. -H. Ludewig, IEEE Trans. Electr. Devices vol. ED-14, p. 816 (1967); G. G. Teather and L. Young, Sol. State Electron Devices vol. 11, p. 527 (1968); J. C. Crawford and F. L. English, IEEE Trans. Electron. Devices vol. ED-16, No. 6, p. 525 (1969)]. Their development slackened however because of obviously insurmountable material problems (badly defined coercive force, fatigue of the switchable charge, slow switching processes, etc.). Such ferroelectrical storage elements also consist basically of a field effect transistor which is placed onto the storage medium. The electrical resistance of the transistor semiconductor layer than depends on the polarization of the electrical field of the ferroelectricum (ferroelectric medium), which in turn is determined by the state of the storage element. However, such constructions elements have the basic disadvantage that the remanent polarization disappears within a few weeks -- so such systems are not immediately suitable for permanent storages. R. R. Metha, B. D. Silverman and J. T. Jacobs, J. Appl. Phys., vol. 44, no. 8, p. 3379 (1973), comes to the conclusion that the depolarization is traceable back to the fact that, because of difference in the postion of the center of mass of the ferroelectrical polarization charges and of the free compensation charges, opposing electrical fields fold back the ferroelectrical domains.